1. Field of the Invention
The present invention relates to a device and to a method for treating items stored in one or more containers, particularly suited to the treatment o silicon wafers placed in storage containers.
It also relates to an apparatus for storing silicon wafers using such a device.
2. Description of the Related Art
Silicon wafers are often stored in containers while they are awaiting production cycles, so as to reduce the effects of molecular contamination of atmospheric origin.
To reduce or eliminate the formation of oxides which is initiated by the presence of water or oxygen molecules near the surface of the wafer, it is known practice for the interior volume of the containers to be inerted by causing a stream of treatment gas, for example nitrogen, to flow through them.
Using this technique, it is necessary to precisely control the flow rate of the treatment gas in the containers, and the pressure of this gas, firstly to effectively eliminate the formation of oxides, and secondly to avoid damage to the containers as this would be likely to contaminate the wafers in so far as the wafers are not designed to withstand pressure variations more than a few tens of millibars relative to atmospheric pressure (both over and underpressure).
There are known, in the prior art, devices for treating items stored in containers, of the type comprising means for supplying the containers with a treatment gas and means for continuously purging the containers, the supply means being equipped with means for regulating the pressure of the treatment gas upstream of the containers.
In this type of device, the flow rate is regulated using mass flow rate controllers based on semiconductor technology. This therefore has rather mediocre reliability in so far as these drift quite often and are liable to exhibit operating faults due to failures of an electrical nature.
Furthermore, such devices are relatively expensive.
The object of the invention is to alleviate these drawbacks and provide a device for treating items which is capable of reliably controlling the flow rate and pressure of the treatment gas in the containers and do so at a relatively low cost.
The subject of the invention is therefore a device of the aforementioned type for treating items, which is characterized in that the purging means comprise a gas ejector connected to the outlet of the containers and supplied with a drive gas so as to regulate the pressure of the treatment gas downstream of the containers and thus control the flow rate of the treatment gas therein.
Thus, the elements used to regulate the flow rate and the pressure of the treatment gas consist of mechanical elements. The flow rate and pressure are therefore controlled with greater reliability.
The device for treating items according to the invention may further have one or more of the following features, taken in isolation or in any technically feasible combination:
the gas ejector constitutes a means for diluting the treatment gas, the flow rate of drive gas constituting a means of controlling the treatment gas content of the gas mixture leaving the ejector;
the ejector comprises two coaxial nozzlesxe2x80x94an internal one and an external onexe2x80x94supplied respectively with drive gas and with treatment gas, the ratio of the passage areas of the nozzles constituting a means for regulating the pressure downstream of the containers;
the means for regulating the pressure of the treatment gas upstream of the containers comprise a pressure reducer inserted between a source supplying treatment gas and the inlet to the containers;
it comprises a pipe running between the inlet and the outlet of the containers and equipped with a passage the internal cross section of which is designed to keep the flow rate of treatment gas in the ejector approximately constant when the number of containers varies; and
it further comprises an upstream-pressure regulator connected to the outlet of the ejector.
Another subject of the invention is an apparatus for storing silicon wafers, comprising a collection of containers for storing the wafers away from the ambient air, characterized in that this apparatus is equipped with a device for treating items as defined hereinabove with a view to treating the wafers, the treatment gas comprising an inert gas, particularly nitrogen or argon, or consisting of purified air.
Another subject of the invention is a method for treating items stored in containers by means of a treatment device as defined hereinabove, the method consisting in supplying the containers with a treatment gas and in continuously purging the containers so as to cause the treatment gas to flow through them, characterized in that the circulation of treatment gas is obtained by regulating the pressure of the treatment gas upstream of the containers, by supplying a gas ejector connected to the outlet of the containers with a drive gas and by regulating at least one parameter chosen from the flow rate and the pressure of the drive gas so as to regulate the pressure of the said treatment gas downstream of the containers and thus control the flow rate of the said treatment gas therein.
According to a particular feature of this method, since the gas mixture leaving the ejector is intended to be discharged into the ambient air, the ratio between the passage areas of coaxial nozzlesxe2x80x94an external one and an internal onexe2x80x94of the ejector, which nozzles are supplied respectively with a treatment gas and with a drive gas, are determined as a function of a maximum ratio between the flow rates of the treatment gas and of the drive gas which corresponds to a maximum treatment gas content leaving the ejector and as a function of the ratio between the pressure of the treatment gas downstream of the containers and the pressure of the drive gas.
As an alternative, since the ejector is intended to be connected, at outlet, to a pipe for extracting the treatment gas, the ratio between the passage areas of coaxial nozzlesxe2x80x94an external one and an internal onexe2x80x94of the ejector, which nozzles are supplied respectively with a treatment gas and with a drive gas, are determined as a function of the maximum ratio between the pressure of the gas mixture leaving the ejector and the pressure of the treatment gas, and the minimum ratio between the flow rates of the treatment gas and of the drive gas is determined as a function of the said pressure ratio.